Effect of corrosive solutions on composites laminates subjected to low

Effects of annealing treatment on the microstructure and mechanical properties of the AlN–SiC–TiB 2ceramic composites prepared by SHS–HIPLijuan Zhou a ,⇑,Hongbo Li b ,Yongting Zheng ba School of Materials Science and Engineering,Shandong University of Technology,ZiBo 255049,PR China bCenter for Composite Materials and Structures,Harbin Institute of Technology,Harbin 150001,PR Chinaa r t i c l e i n f o Article history:Received 14July 2012Received in revised form 10November 2012Accepted 12November 2012Available online 20November 2012Keywords:AlN–SiC–TiB 2ceramic Annealing treatment MicrostructureMechanical propertiesa b s t r a c tAnnealing treatments were carried out at different temperatures (600–1200°C)for various holding times (2–10h)to evaluate the microstructural and mechanical properties changes of the AlN–SiC–TiB 2ceramic composites prepared by SHS–HIP.The experimental results show that the annealing treatment is bene-ficial to improve the mechanical properties.In the meantime,the desolution of SiC-rich solid solution and precipitation of fine TiB 2grains occurred.Due to the precipitation strengthening by the SiC-rich solid solution and the grain boundary strengthening by fine TiB 2particles,the improvement of the mechanical properties was obvious with higher annealing temperatures and longer holding times.The highest HRA,bending strength and fracture toughness were 94.1,517.5MPa and 5.79MPa m 1/2,respectively,after the annealing treatment at 1200°C for 10h.Ó2012Elsevier B.V.All rights reserved.1.IntroductionAlN–SiC solid solution has a wide range of engineering appli-cations due to their excellent mechanical properties at high tem-peratures and has been investigated considerably in the past decades years [1,2].Many preparation methods,such as hot pressing [3],reaction synthesis [4],spark plasma synthesis sinter-ing [5]and combustion synthesis [6],were adopted to fabricate AlN–SiC ceramics.TiB 2is another important material for high temperature applications because of its high melting point,mechanical properties and electrical conductivity,and relatively low coefficient of thermal expansion.So AlN–SiC–TiB 2ceramics have wide potential applications in the field of electrical conduc-tivity at high temperatures.It is known that combustion synthesis (self-propagating high-temperature synthesis,SHS)is a novel method for preparation of AlN–SiC–TiB 2ceramics with many advantages,such as high efficiency and energy saving.However,combustion synthesis is a high temperature and velocity reaction process,which often result in large amount of non-equilibrium phases and high residual stress,and this deteriorates the proper-ties of the combustion synthesized products.So annealing treat-ment at elevated temperatures (800–1200°C)has been introduced to reduce the residual thermal stress and improve the mechanical properties [7,8].In this paper,the effects of annealing treatment on properties of the AlN–SiC–TiB 2ceramics prepared by self-propagating high tem-perature synthesis and hot isostatic pressing (SHS–HIP)were stud-ied.The influences of the annealing temperature and holding time on the composition,microstructure and mechanical properties were discussed in detail.2.Experimental procedureThe AlN–SiC–TiB 2ceramics were prepared by SHS–HIP reported in reference [9].The reaction for preparing AlN–SiC–TiB 2ceramics is as follows:Al þSiC þTiB 2þN 2!AlN þSiC þTiB 2ð1ÞThe samples with Al 35wt%–SiC 35wt%–TiB 230wt%were chosen to carry out the annealing treatment tests.The annealing treatments were carried out on the box-type electric furnace (SXK-8-16,Longjiang electrical furnace works,Harbin,China)in vacuum atmosphere.The specimens were enclosed and vacuumed in quartz glass tube.During the annealing treatment test,four different temperatures (600,800,1000and 1200°C)and five different holding times (2,4,6,8and 10h)were adopted to study the effects of annealing temperature and holding time on the microstructure and mechanical properties of the AlN–SiC–TiB 2ceramics.The composition of the annealing treated samples was analyzed by X-ray dif-fraction (D/max-rB,Rigaku,Japan).The microstructures were observed by scanning electron microscope (SEM,HITACHI S-4700),energy dispersive X-ray spectrometer,and transmission electron microscopy (TEM,Philips CM12/STEM,Holland).The specimen of 3mm Â4mm Â36mm and 2mm Â4mm Â22mm in dimension for bending strength and fracture toughness tests were sliced respectively.The three-point bending strength and fracture toughness were tested on electronic uni-versal test machine with crosshead speed of 0.5mm/min (Instron5569,USA).The Rockwell hardness test was carried out with a ½00ball and 60kg load applied for 30s on the Rockwell hardness test machine (HR-150A,China).0925-8388/$-see front matter Ó2012Elsevier B.V.All rights reserved./10.1016/j.jallcom.2012.11.080Corresponding author.Tel./fax:+865332781357.E-mail address:zhoulijuan@ (L.Zhou).3.Results and discussion3.1.MicrostructureFig.1shows the XRD patterns of theafter the annealing treatment for10h atAs thefigure indicated,oxidationnot occurred and the main phases wereTiB2after the annealing treatment.temperature and nitrogen pressure duringcontent of hexagonal boron nitride(h-BN)ucts.With the increasing of the annealingtion intensity of h-BN phase decreased,TiN phase appeared at the annealing10h,and slightly increases as the annealingto1200°C.The formation of the smallfavorable to the improvement of thermalfracture toughness because of theirand micro-cracking interactionkind of structural ceramic with excellenttion of h-BN phase during preparation anding treatment will not decrease the properties of the products,onthe contrary,it is helpful to the improvement of mechanical properties.Fig.2shows the XRD patterns of AlN–SiC–TiB2ceramics after annealing treatment at1200°C for different holding times.It can be found that the composition of the annealing treated ceramics was almost unchanged at1200°C for8h.When the holding time increased to10h,a small quantity of TiN was formed in the prod-uct,which was consistent with the XRD results shown in Fig.1. Furthermore,the X-ray diffraction peaks of2H–AlN–SiC phase were becoming narrower and sharper with the increasing of the annealing holding time.Meantime,the XRD peaks of the2H–AlN–SiC phase appeared to shift to larger angles,which indicated 3.2.Strengthening and toughening mechanismFig.3shows the BSE images of AlN–SiC–TiB2samples annealed for10h at600and1200°C.AlN–SiC matrix shows gray color and the dispersed white particles are TiB2phase.Fig.3a shows the sam-ple surface annealed at600°C for10h.White columnar TiB2were dispersed inhomogenously in the matrix,and also the sample sur-face presented many pores with different sizes and shapes.When the annealed temperature was1200°C,the dispersion of TiB2 was relatively homogenous andfiner,and the porosity of the sur-face was relatively decreased,as shown in Fig.3b.It is noted that some light gray phase exists between gray ma-trix and white particles,as shown in Fig.4.Line scanning for the light gray region was conducted to determine the phase composi-tion,as shown in Fig.4.The results show that the light gray phase contained large amount of Si element and tiny content of Al,and no Ti element was detected,which can be indicated that the light gray phase was the SiC-rich phase precipitated during the annealing treatment.After the precipitation of SiC,the AlN-rich and SiC-rich solid solutions were formed correspondingly,which was consis-tent with the phase diagram analysis[10].Phase separation leads to grain size refinement,and then results in the improvement of the yield strength.According to Hall–Petch equation,the relationship between the yield strength and grain size is described as follows[11]:ry¼r0þkdÀ1=2ð2Þwhere r y is the yield strength,r0is a materials constant for the starting stress for dislocation movement,k is the strengthening coefficient,and d is the average grain diameter.As the equation indicated,the grain size refinement is in favor of the improvement of yield strength of the materials.Fig.5shows the TEM image and SAED pattern of AlN–SiC–TiB2 ceramic annealed at1200°C for10h.Light gray matrix was AlN–SiC and the black particles embedded in the matrix were TiB2 phase,as shown in Fig.5a.The SAED pattern of the TiB2particles and the calibration were shown in Fig.5b and c,respectively.The average size of these TiB2particles was much less than those in reactant,and also,these particles were completely embedded in the AlN–SiC matrix,which indicated that thefine TiB2particles were precipitated during the annealing treatment.Since combustion synthesis is a high temperature and velocity reaction process,many dislocations,grain boundaries andFig.1.XRD patterns of AlN–SiC–TiB2ceramics annealed at different temperatures for10h.Fig.2.XRD patterns of AlN–SiC–TiB2ceramics annealed at1200°C for different holding times.500L.metastructures can be formed in the products.In addition,boron and titanium could dissolve in the AlN–SiC matrix or in the grain boundaries because of high reaction temperature.What is more,the standard enthalpy of formation (D H )of TiB 2is relatively low (D H =À293kJ mol À1),and the fine TiB 2particles can be precipi-tated from the supersaturated and non-equilibrium AlN–SiC ma-trix during the high temperature annealing treatment.Precipitating of these fine TiB 2particles decreased the lattice distortion,which also released the inner stress and improved the order degree of the lattice.Meantime,composition gradient formed by the precipitation between the matrix and TiB 2phases enhanced the interface bonding strength.In summary,the improvement of the strength and toughness of the AlN–SiC–TiB 2ceramics can be attributed to the particle dis-persion strengthening effect of the precipitated SiC-rich solid solution.And also the fine TiB 2particles precipitated uniformly at the phase interface enhanced the strength of the grain boundaries.nano-particles in AlN–SiC–TiB 2ceramic annealed at 1200°C for 10h:(a)TEM image of AlN–SiC–TiB 2composite;(b)SAED of light gray phase in the AlN–SiC–TiB 2ceramic annealed at 1200°C for 10h:(a)SEM image of the surface;(b)line Fig.3.Backscattered electron images of AlN–SiC–TiB 2samples annealed at (a)600°C,10h;(b)1200°C,10Fig. 6.Typical SEM fractograph in annealing treated AlN–SiC–TiB2ceramic composite.Fig.7.Rockwell hardness of AlN–SiC–TiB2ceramics annealed at different temper-atures for various holding times.Fig.8.Bending strength of AlN–SiC–TiB2ceramics annealed at different temper-atures for various holding times.Fig.9.Fracture toughness of AlN–SiC–TiB2ceramics annealed at different temper-atures for various holding times.annealing temperature reached800and1200°C.The highest HRA,bending strength and fracture toughness were94.1, 517.5MPa and 5.79MPa m1/2,respectively,after the annealing treatment at1200°C for10h.4.ConclusionsThe effects of annealing treatments on the microstructure and mechanical properties of AlN–SiC–TiB2ceramics prepared by SHS–HIP were studied.The annealing treatment can promote the precipitation SiC-rich solid solution in AlN–SiC matrix andfine TiB2particles on the grain boundaries,which improve the mechan-ical properties obviously.With the annealing temperature increas-ing from600to1200°C and holding times increasing from2to 10h,the strengthening effect was more evident.The highest HRA,bending strength and fracture toughness of AlN–SiC–TiB2 ceramics were94.1,517.5MPa and5.79MPa m1/2,respectively, after the annealing treatment at1200°C for10h.AcknowledgmentsThis work has beenfinancially supported by‘‘A Project of Shandong Province Higher Educational Science and Technology Program(J09LD01)’’and‘‘Shandong Province Natural Science Foundation(ZR2010EM045)’’.References[1]L.J.Zhou,Y.T.Zheng,S.Y.Du,H.B.Li,Mater.Sci.Forum546–549(2007)1505–1508.[2]vrenko,J.Desmaison,A.D.Panasyuk,M.Desmaison-Brut,E.Fenard,J.Eur.Ceram.Soc.25(2005)1781–1787.[3]K.Strecker,M.J.Hoffmann,J.Eur.Ceram.Soc.25(2005)801–807.[4]X.M.Yue,G.J.Zhang,Y.M.Wang,J.Eur.Ceram.Soc.19(1999)293–298.[5]M.Hotta,J.Hojo,J.Eur.Ceram.Soc.30(2010)2117–2122.[6]L.Mei,J.T.Li,Acta Mater.56(2008)3543–3549.[7]A.Varma,A.S.Rogachev,A.S.Mukasyan,S.Hwang,Adv.Chem.Eng.24(1998)79–226.[8]D.Sciti,S.Guicciardi,A.Bellosi,J.Eur.Ceram.Soc.21(2001)621–632.[9]L.J.Zhou,Y.T.Zheng,S.Y.Du,Key Eng.Mater.353–358(2007)1517–1520.[10]A.Zangvil,R.Ruh,J.Am.Ceram.Soc.71(1988)884–890.[11]H.Conrad,J.Narayan,Scripta Mater.42(2000)1025–1030.L.Zhou et al./Journal of Alloys and Compounds552(2013)499–503503。

小学上册英语第二单元期末试卷英语试题一、综合题(本题有100小题，每小题1分，共100分.每小题不选、错误，均不给分)1.The capital city of Nigeria is __________.2.My uncle loves to __________. (旅行)3. A frog can change its color based on its ______ (环境).4.What is the name of the famous scientist known for his contributions to the field of genetics?A. Gregor MendelB. Charles DarwinC. Louis PasteurD. Albert EinsteinA5.The _______ (猪) will oink when happy.6.What is the name of the first living creature in space?A. LaikaB. BelkaC. StrelkaD. Yuri7.We have _____ (a/an) apple for snack.8.What do we call the time when the sun rises?A. MorningB. NoonC. EveningD. Midnight9.Kale is a type of ______ (绿叶菜).10.Dolphins are very _______ and friendly.11.My friend is very ____.12. A _______ is a type of solution that contains more solute than it usually can at a given temperature.13.How many wheels does a bicycle have?A. OneB. TwoC. ThreeD. Four14.What do you call a person who sings songs?A. DancerB. MusicianC. SingerD. PerformerC15.We measure temperature in degrees ______.16.What is the name of the famous clock tower in London?A. Big BenB. Eiffel TowerC. Leaning TowerD. Tower of PisaA17.My toy dinosaur is very ______.18.In a chemical equation, the products are shown on the _____ side.19.I like to _______ (jump) rope.20.This boy, ______ (这个男孩), loves to ride his bike.21.What do we call a person who repairs cars?A. DoctorB. MechanicC. PilotD. Chef22.The ______ is the part of a flower that produces seeds.23.We should respect all _____ (自然环境).24.At the pet shop, I saw a _______ (小仓鼠) running in a wheel.25.I enjoy _______ (参加) community service.26.I want to ________ (visit) my grandma.27.The soup is ___ (hot/cold) today.28.What is the name of the famous American singer known for "All of Me"?A. John LegendB. Ed SheeranC. Sam SmithD. Michael BubléA29.All matter is made up of tiny particles called ______.30.What is the name of the famous American singer known for "Jolene"?A. Reba McEntireB. Dolly PartonC. Carrie UnderwoodD. Miranda LambertB31.Which of these is NOT a type of fruit?A. AppleB. OrangeC. CarrotD. BananaC32.The country known for its multicultural societies is ________ (以多元文化社会闻名的国家是________).33.The parrot can _________ words. (模仿)34.The Earth's atmosphere is made up of different ______ gases.35.My sister likes to sing __________. (歌曲)36. A covalent bond is formed when atoms __________ electrons.37.I have a collection of miniature ________ (玩具类型).38.Chemical changes can result in the release of ________.39.My uncle is a fantastic ____ (storyteller).40.The country that is famous for pyramids is __________.41.They are _______ (building) a sandcastle.42.I made a ______ (贺卡) for my friend's birthday. She was very ______ (感动).43. A ____ is a small animal that loves to forage for food.44. A lion is a brave _______ that roams the savanna.45.Orbiting space debris poses a risk to ______.46.What is the main ingredient in a salad?A. RiceB. VegetablesC. MeatD. Bread47.The __________ (历史的启示性探讨) foster understanding.48. A solid has a definite ______ and volume.49.Polyatomic ions consist of multiple ______.50.The _______ (小浣熊) washes its food before eating.51. A seahorse is a type of ______ (鱼).52._____ (花园) can be enjoyed by everyone.53. A ________ (水獺) loves to swim and catch fish.54.What do we call a person who studies the past?A. HistorianB. ArchaeologistC. AnthropologistD. SociologistA55.What do we call the process of taking care of plants?A. GardeningB. FarmingC. CultivationD. All of the aboveD All of the above56.She is a _____ (作家) writing for various platforms.57.The __________ (社会变革) can lead to progress.58. A chemical reaction that releases heat is called an ________ reaction.59.Which of these is a warm-blooded animal?A. FishB. BirdC. LizardD. FrogB60.What do we call a piece of furniture we sit on?A. TableB. ChairC. BedD. Couch61.The chemical symbol for scandium is ______.62.The _____ (章鱼) can squeeze into tight spaces.63.My favorite dish is ______ (沙拉).64.The Earth's surface is shaped by a combination of internal and ______ forces.65.What is the main ingredient in lemonade?A. WaterB. SugarC. LemonD. All of the aboveD66.My mom is a __________ (会计师).67.The _______ (猪) rolls in mud.68.I saw a _______ (小鸟) perched on a tree.69.The __________ (历史的共同创造) enriches narratives.70.What do we call a person who studies the effects of climate change?A. ClimatologistB. MeteorologistC. Environmental ScientistD. EcologistA71.The quokka is known as the world's ________________ (最快乐) animal.72.What is the name of the spacecraft that took humans to the moon?A. DiscoveryB. ApolloC. GeminiD. Voyager73.What is the main source of energy for all living things?A. WaterB. FoodC. SunlightD. AirC Sunlight74.The __________ (历史的深刻) inspires thought.75.I enjoy cooking ________ with my dad.76.She is a great ___. (helper)77.The cake is _____ with icing. (covered)78.The __________ (印度独立) movement was led by Mahatma Gandhi.79.小老鼠) nibbles on cheese. The ___80.The __________ is a famous mountain range in Asia. (喜马拉雅山)81.The __________ is the process of breaking down large molecules into smaller ones.82.The cake is very ______ (sweet).83.Which day comes after Monday?A. SundayB. TuesdayC. WednesdayD. ThursdayB84.What do we call the act of studying the past?A. ArchaeologyB. HistoryC. AnthropologyD. Paleontology85.What do we call the device used to look at distant objects?A. MicroscopeB. TelescopeC. PeriscopeD. Kaleidoscope86.The ant works together in a _______ (群体).87.The chemical formula for -pentanol is ______.88.In winter, I wear ______ (靴子) to keep my feet warm.89.What is the capital of Mexico?A. Mexico CityB. GuadalajaraC. MonterreyD. PueblaA90.What is the capital of Sweden?A. OsloB. CopenhagenC. HelsinkiD. StockholmD91.What do we call the application of mathematics to solve real-world problems?A. AlgebraB. GeometryC. CalculusD. Applied MathematicsD92.I want to _____ (learn/teach) English.93.What is the capital of China?A. BeijingB. ShanghaiC. Hong KongD. TaipeiA94.The painter, ______ (画家), creates beautiful art.95.My family has a ______ (小狗) and a ______ (小猫). They are best ______ (朋友).96.I want to _______ (学习) how to fish.97.Which fruit is known for having seeds on the outside?A. AppleB. StrawberryC. BananaD. CherryB98.The ____ is often seen fluttering around flowers.99.I want to learn how to ________ (做手工艺).100.My school has a big ________ (图书馆) filled with many interesting ________ (书籍).。

Improved corrosion protection of magnesium by hydrothermally deposited biodegradable calcium phosphate coatingSara Kaabi Falahieh Asl a ,b ,Sandor Nemeth b ,*,Ming Jen Tan aa Nanyang Technological University,School of Mechanical &Aerospace Engineering,50Nanyang Avenue,639798,Singapore bSingapore Institute of Manufacturing Technology,71Nanyang Drive,638075,Singaporeh i g h l i g h t sThe corrosion protection of hydrothermally deposited Ca e P coatings was studied.The Ca e P coatings improved the corrosion performance of Mg substrate up to 10,000-fold. The corrosion resistance improved by increasing deposition temperature.Deposition mechanism of coating changed by increasing deposition temperature.a r t i c l e i n f oArticle history:Received 4July 2014Received in revised form 19February 2015Accepted 13May 2015Available online 23May 2015Keywords:Biomaterials Coatings CorrosionCorrosion testElectrochemical techniques Electrochemical propertiesa b s t r a c tThe corrosion protection of hydrothermally deposited calcium phosphate coatings on AZ31magnesium alloy were studied for their potential use in biocompatible and bioresorbable temporary implants.The coatings mainly consisted of calcium phosphate phases (monetite and tricalcium phosphate).Poten-tiodynamic and electrochemical impedance spectroscopy (EIS)con firmed that the coatings provided varying levels of corrosion protection depending on the coating deposition temperature and duration.EIS results showed that the size of capacitance loops and the absolute impedance value (j Z j )increases by increasing the deposition temperature and corresponding growth in coating thickness.In agreement with the electrochemical experiments,immersion tests in simulated body fluid also indicated large improvement in corrosion protection as the mass loss was signi ficantly reduced when coating was applied as compared to the bare ing the thickest coating obtained at 190 C deposition tem-perature,the corrosion current density of the coated magnesium was 10,000fold lower compared to the bare metal.This result con firmed that the new hydrothermal coating is suitable to protect Mg implant against corrosion with further advantage of being bioactive and biodegradable.©2015Elsevier B.V.All rights reserved.1.IntroductionThe interest in using magnesium alloys has been steadily increasing [1e 3]due to their low density and favorable mechanical properties.However,a signi ficant obstacle of using magnesium in most applications is its rapid corrosion requiring corrosion pro-tection measures.One of the emerging applications of magnesium is its use as biocompatible and biodegradable temporary implant material [2,4],when its corrosion is taken advantage of by allowing the magnesium metal to corrode and be absorbed by the body.Magnesium could provide signi ficant advantage compared tocurrently used implants made of titanium (Ti),stainless steel (S.S.),and cobalt e chromium (Co e Cr)alloys when used for temporary implantation since it does not require removal and associated second operation.Magnesium alloys are also advantageous in reducing the stress shielding effect due to the mismatch in me-chanical properties between implant materials and bone tissue.The stress shielding phenomenon causes stress concentration at bone-implant interfaces and may result in critical clinical issues such as implant loosening,delay in healing process and consequently damaged bone growth.Ti,SS,and Co e Cr alloys have high Young's modulus (100e 200GPa)while magnesium alloys have lower modulus (41e 45GPa),close to that of human bone (10e 30GPa);therefore use of Mg avoids or minimizes stress shielding effect [2,4,5].Despite the high potential of using magnesium alloys as*Corresponding author.E-mail address:sandorn@.sg (S.Nemeth).Contents lists available at ScienceDirectMaterials Chemistry and Physicsjo urn al homepag e:/locate/matchemphys/10.1016/j.matchemphys.2015.05.0350254-0584/©2015Elsevier B.V.All rights reserved.Materials Chemistry and Physics 161(2015)185e 193temporary and biodegradable implants,applications are not widespread as the corrosion rate of Mg alloys is too high,resulting in excessive hydrogen evolution and local alkalization close to the surgery region,as well as premature degradation in the implant's mechanical integrity before sufficient bone healing can occur[3].To put Mg to practical use,it is necessary to improve its corrosion performance through alloying or surface modification.Although the corrosion resistance of Mg can be enhanced by alloying ele-ments with low toxicity such as zinc and calcium,as well as the potentially more harmful aluminum or rare earth metals,the amounts of these elements must be limited in order to maintain desired mechanical properties and reduce potential toxicity leading to insufficient improvement in corrosion properties.The alternative is surface modification and coating that could reduce the corrosion rate to a higher extent[6,7].Anodizing,electrodeposition,fluoride conversion and biomimetic coatings have been used on Mg alloys. However,these coatings do not provide sufficient protection:Bio-mimetic and anodic coatings are reportedly rather permeable, exhibiting many cracks[1,8].Relatively thin but dense coating can be made of magnesiumfluoride(MgF2)as a conversion coating but it gradually dissolves when the sample is transferred tofluoride-free solutions;precluding long-term protection[1,8].Thus,no suitable coating seems to exist for magnesium implant application and there is a need for a novel coating that is dense and able to provide superior corrosion protection.In addition,the coating must be biodegradable and biocompatible as the biological response of implant materials strongly depends on the implant's surface properties due to interactions between the cells and biomaterials taking place at the tissue e implant interface[9].Calcium-phosphate(Ca e P)coatings are preferred for biomed-ical applications since Ca e P is the main inorganic constituent of natural bone and it can speed up bone growth.Producing Ca e P crystals by hydrothermal process has a successful history[10e14]. However,there are only a few publications regarding deposition of Ca e P crystals directly on metal substrate.Liu et al.reported deposition of a fairly uniform crystalline hydroxyapatite(HA)layer on metal substrates by seeded hydrothermal deposition method in two steps:Deposition of a seed layer by means of electrochemical method which was followed by hydrothermal process.They found that the seed layer improves the deposition of HA layer during hydrothermal process.However,they did not investigate the corrosion performance of the obtained coatings[15].Onoki et al. used a double layered capsule hydrothermal hot-pressing tech-nique for developing HA coating on Ti and Mg substrates with good adhesion properties(4e5.5MPa)[16,17].Hiromoto et al.[18]used hydrothermal method to deposit HA in a wide range of pH(6.3,7.3, 11.3)at8h and24h deposition times.Longer deposition time and near neutral pH enhanced the coating performance and they re-ported a tenfold improvement in the corrosion rate reduction of Mg substrate based on corrosion current densities.However,this improvement is not fully adequate for protecting magnesium im-plants in applications where the healing may take several months as there is potential for rapid loss of mechanical integrity of the metal through corrosion.Ng et al.[19]and Zhu et al.[20]developed a thick magnesium hydroxide layer,Mg(OH)2,by means of hydro-thermal process for surface modification of Mg.Gupta et al. developed compact and pore-free Mg(OH)2layer which improved corrosion performance of Mg substrate in phosphate buffer saline (PBS)solution[21].Although,Mg(OH)2layer can improve the corrosion performance of Mg substrate it is not preferred in biomedical application unlike the biodegradable and biocompat-ible Ca e P coatings.Apart from biocompatibility requirement,the deposited coating layer must be dense and adherent.Thus,a hy-drothermal deposition method was selected in the present work to grow bioresorbable Ca e P coating in such a way that most of the coating defects would undergo self-healing during the deposition and thus provide high corrosion resistance at least for the initial healing period when mechanical support is needed.The hydrothermally deposited Ca e P coating was successfully prepared and partially characterized for its morphology,thickness, composition and mechanical properties as reported separately[22]. According to this report,XRD analysis showed that the deposits obtained from hydrothermal process were mainly a mixture of monetite(CaHPO4)and tricalcium phosphate with small amount of magnesium substitution[Ca2.86Mg0.14(PO4)2,also known as whit-lockite].Similarly,the FTIR spectra for Ca e P coating layers depos-ited showed vibration bands corresponding to monetite and tricalcium phosphate phases.Moreover,the coating thickness was found to increase steadily by increasing the deposition temperature from100 C to160 C with3h deposition time(20m m at100 C, 48m m at130 C,61m m at160 C)followed by a sharp increase to 389m m at190 C.With the increase in coating thickness,the coating adhesion dropped from5.8MPa(sample coated at160 C) to3.1MPa(sample coated at190 C).The investigation of corrosion behavior of the coated magnesium alloy by electrochemical methods is reported in this work.2.Experimental details2.1.MaterialsThe hydrothermal coating deposition method was reported elsewhere[22],using analytical grade Ca(NO3)2(99.98%purity)and NH4H2PO4(98%purity),obtained from Alfa Aesar.AZ31magnesium alloy was used as substrate for the coating process.Disks were prepared with25mm diameter and2mm thickness from com-mercial AZ31rod.Prior to coating process,AZ31substrate was prepared with up to2400grit abrasive paper,and then ultrasoni-cally cleaned in acetone for20min prior to coating.The coated samples were named as HE100ºC-3hr and HE130ºC-3hr,HE160 C-3hr and HE190 C-3hr respectively,referring to the deposition temperature and duration.For the190 C hydrothermal process, heating durations of1and2h were used as well.2.2.Characterization of coatingsThe electrochemical behavior of uncoated and coated samples was investigated on duplicate samples.Potentiodynamic polariza-tion tests were done using a Solarton potentiostat(Model SI1287) conducted in simulated bodyfluid solution(SBF)according to a reported procedure[23]at pH7.2e7.4.For the electrochemical tests,a three-electrode cell was used with the sample as working electrode,SCE(sat.KCl)was used as the reference,and graphite as the counter electrode.An area of1cm2of the working electrode was exposed to the solution.A1mVsÀ1scanning rate was applied during the potentiodynamic polarization test.Prior to the polari-zation test,samples were equilibrated in SBF solution for30min.Electrochemical impedance spectroscopy(EIS)test was per-formed in order to investigate the corrosion behavior of coatings in SBF solution using a Solarton potentiostat(Model SI1260).The EIS tests were conducted at the open circuit potential(OCP)in the frequency range of100kHz to10mHz,using10mV amplitude of signal's perturbation.Immersion tests were conducted in SBF standard solution for up to28days in order to assess the early corrosion performance.Each sample with an exposure area of4.9cm2was immersed in50ml of the SBF solution at37 C.The exposure area was controlled by covering the rest of the sample surface with a polymer resin.After each immersion time,the samples were extracted and the corro-sion products were removed by using chromic acid and silverS.Kaabi Falahieh Asl et al./Materials Chemistry and Physics161(2015)185e193 186nitrate according to the method described in the literature [24];then the samples were cleaned with ethanol and dried.Finally,the sample masses were measured.After each immersion period in SBF solution,5ml of solution was extracted and analyzed by using an inductively coupled plasma atomic emission spectrometer (ICP-MS7700,Agilent)in order to determine the Mg ion release from the samples during the im-mersion periods.The changes in pH value of the SBF solution with uncoated and coated samples were monitored overtime as well.For the immersion test and ion release measurements,the reported results are the means of triplicate measurements and the data are reported as mean ±standard deviation.3.Result and discussion 3.1.Electrochemical test resultsElectrochemical measurements were done in SBF solution to characterize the corrosion performance of the samples by means of open circuit potential (OCP)and potentiodynamic polarization.The variation of OCP over time can be used as a criterion to understand the interface stability of coating and substrate.OCP curves foruncoated and coated samples are illustrated in Fig.1a.The OCP of the coated samples shifted to nobler potential as compared to the bare alloy.The OCP of coated sample deposited at 190 C increased by 1.72V compared to AZ31bare substrate while 1.22V increase was seen for coated sample at 160 C.The positive shift of OCP due to the presence of coating indicates the formation of a stable Ca e P/AZ31interface [25,26].The much nobler behavior of the coated sample at 190 C and the stable OCP over a 30min period can be attributed to the improved stability of the coated alloy surface that may be related to high coating thickness and more complicated paths of electrolyte penetration into coating layer due to its compact morphology.The observations indicated that Ca e P layer has a protective nature and could provide good protection for Mg substrate.Figs.1b and 2show the potentiodynamic curves for coated samples deposited at varied temperatures for three hours and at 190 C for different deposition times,respectively.The corrosion potential (E corr )and the corrosion current densities (j corr )were derived from the polarization curves by means of Tafel extrapola-tion method,and the results are summarized in Table 1.According to the data,the corrosion rate of coated samples decreased signif-icantly compared to the uncoated sample.The corrosion rate was reduced by increasing the hydrothermal deposition process tem-perature.This was likely due to an increase in the coating thickness and changes in coating morphology.However,the corrosion resistance did not change uniformly with coating thickness.Plot-ting log[j corr /(A cm À2)]from Table 1against coating thickness gives a linear trend for the bare substrate and samples deposited in the 100 C to 160 C temperature range for 3h but the sample made at 190 C does not fall in the same trend.The trendline has a slope of À0.0455/m m and constant of À3.90.The trend would predict 84m m thickness for the HE190 C-3hr sample at its observed corrosion current density while the measured coating thickness is 389m m.This suggests that the coating structure in the highest deposition temperature conditions may not be the same as that of the other samples.Although the corrosion resistance of Mg sub-strate was enhanced approximately 10,000-fold after coating at 190 C,the coating deposited at this high temperature appears to be less protective for equivalent thickness than the coatings made at lower temperatures.Mg and its alloys are known to suffer from pittingcorrosionFig.1.a)Open circuit potential curves and b)potentiodynamic curves of uncoated and coated samples for different deposition temperature in SBFsolution.Fig.2.Potentiodynamic curves of uncoated and coated samples at 190 C for different deposition time.S.Kaabi Falahieh Asl et al./Materials Chemistry and Physics 161(2015)185e 193187manifesting in rapid increase in the anodic current over small po-larization potential range in potentiodynamic scans which can be seen clearly in Fig.1b for the bare substrate and coated sample at 100 C.On the other hand,samples coated at 130 C and 160 C,the polarization curves show an abrupt increase in anodic current fol-lowed by a quasi-passive region at around 10À5A/cm 2,while in the case of the coated sample at 190 C it only had a quasi-passive region in polarization curve at around 5*10À7A/cm 2.The change in corrosion rate of the sample coated at 190 C was examined for different deposition durations from 1to 3h,as shown in Fig.2.By increasing the deposition time,the protective nature of the coating improved,probably due to the increase in coating thickness and formation of a more compact morphology.Plotting log[jcorr/(A.cm À2)]from Table 1against coating thickness for samples deposited at 190 C also showed a linear trend with slope of À0.00937/m m and constant of À4.00.This slope was much less steep than the one found for samples deposited at lower temper-atures and appears to indicate a lower corrosion protection for equivalent coating thickness when deposition is done at 190 C.The corrosion improvement can be better understood by considering the corrosion process of the metal.Mg reacts with SBF solution immediately upon contact and starts to corrode according to Eq's.(1e 3)[27]:Mg /Mg 2þþ2e À(1)2H 2O þ2e À/H 2þ2OH À(2)Mg 2þþ2OH À/Mg ðOH Þ2(3)The presence of a coating layer can prevent or delay the access of electrolyte to the Mg substrate and retard the initiation of corrosion process by limiting the availability of water needed for hydrogen generation according to Eq.(2).Thus,by increasing the coating thickness and improving the coating morphology,the corrosion rate can be decreased [26].3.2.Electrochemical impedance spectroscopy (EIS)resultsEIS measurements were done to understand the corrosion behavior and corrosion performance of coated and uncoated sub-strate in SBF solution.EIS measurements were analyzed by two methods:Nyquist impedance spectroscopy and Bode graphs for the bare and coated samples as given in Figs.3and 4,respectively.It was clearly observed that the Nyquist graph for bare sample composed of two capacitive semi-circuits at medium and low fre-quency,followed by an inductive loop at low frequency region.The Bode phase graph of bare sample showed two phase maxima at low and medium frequency.The time constant at medium frequency can be attributed to the partially protective oxide layer and semi-circle at low frequency might be related to the charge transfer resistance (R ct )and double layer capacitance (C dl )at the surface ofsubstrate [28].The calculated results from EIS graphs are reported in Table 2.The appearance of inductive loop at low frequency is probably due to film weakening and pit incubation.Coated samples at 100 C,130 C and 160 C only showed a capacitance loop at medium frequency.The capacitive loop at medium frequency might be attributed to the coating layer polarization resistance (R p ).The calculated capacitances for these time constants were in the range of 4e 12m F.Since the double layer capacitance is normally between 20and 50m F [29],the appearance of this semi-circle is related to coating layer resistance.The size of capacitance loops was increased by increasing the deposition temperature.This is in parallel to the increase in charge transfer and polarization resistance which is a criterion for corrosion resistance enhancement [30].The sample coated at 190 C showed a different behavior.The Nyquist graph was composed of two capacitive loops at high and medium frequency.The high frequency time constant can be related to coating layer resistance and the presence of capacitive loop at medium frequency range can be related to the new surface formation at the electrode surface as a result of partial delamina-tion of top surface of coating [31].This result is in agreement with the potentiodynamic test data.According to Fig.1a and b,there is aTable 1Coating thickness and potentiodynamic polarization curve parameters derived by the Tafel extrapolation.Sample Coating thickness (m m)j corr (A cm À2)E corr (V vs.SCE)AZ31substrate 0 1.2*10À4À1.5HE100 C-3hr 20 1.2*10À5À1.5HE130 C-3hr 48 2.8*10À6À1.1HE160 C-3hr 61 1.1*10À7À0.8HE190 C-3hr 389 2.3*10À8À0.7HE190 C-2hr 150 4.2*10À6À0.7HE190 C-1hr582.2*10À5À1.4Fig.3.a)Nyquist plot of uncoated AZ31(appears near the origin only)and coated samples in SBF solution,b)Enlarged Nyquist plot of uncoated AZ31from plot a)to show detail.S.Kaabi Falahieh Asl et al./Materials Chemistry and Physics 161(2015)185e 193188big difference between the OCP value for the coated sample at 190 C and its corrosion potential.Normally the values of these two potentials are close.However,the formation of a new surface during corrosion process can result in the shift of these potentials.Since the coating made at 190 C was the thickest,the high thickness might result in increased residual stress and delamina-tion of the top layer of coating resulting in the emergence of the second capacitive loop at medium frequency range.However,for the coated sample at190 C,the diameter of capacitive loop at medium frequency,was signi ficantly larger than that of other coatings,which con firmed that this coating possesses higher corrosion resistance compared to other coatings.From the Bode plot (Fig.4b),it is obvious that coated substrates have larger impedance (j Z j )values compared to the bare substrate,which means the corrosion performance of coated sample is much better than bare substrate [26].The absolute impedance value (j Z j )increases by increasing the deposition temperature.The EIS results are thus in good agreement with potentiodynamic results and con firmed that by increasing deposition temperature,the corrosion performance of coated samples improved.The impedance data for the bare and coated samples were fitted to theoretical models as shown in Fig.5.For the bare substrate,C f is considered as oxide film capacitance,while in the case of the coated sample it is related to the coating layer capacitance.C ns is related to the newly formed surface at the surface of coated sample at 190 C.R p and R pns represent the coating layer and newly formed surface polarization resistance respectively.R ct represents the charge transfer resistance at the surface of substrate and C dl shows the double layer capacitance at the surface of electrode and L repre-sents an inductance.The fitting results were in good agreement with the original impedance graphs.3.3.Mass loss of samples during immersion testThe mass loss is a common test for evaluating corrosion per-formance as reported in literature [32,33].In this study the massTable 2EIS parameters derived from impedance spectra.Sample R ct(U .cm 2)R p(U .cm 2)R ns(U .cm 2)j Z j(U .cm 2)C dl(m F/cm 2)C f(m F/cm 2)C ns(nF/cm 2)AZ31substrate 47.161.49…14120.68.2…HE100 C-3hr …1981…1421… 3.9…HE130 C-3hr …1259.7…928.9… 4.1…HE160 C-3hr …4182…3305…9.6…HE190 C-3hr…6478.927487712…12.30.58Fig.5.Equivalent circuits for:a)uncoated substrate,b)coated samples at 100 C,130 C and 160 C and c)coated sample at 190C.Fig.4.a)Bode phase graph and b)Bode plot graph for uncoated and coated samples in SBF solution.S.Kaabi Falahieh Asl et al./Materials Chemistry and Physics 161(2015)185e 193189loss of bare and coated magnesium samples was monitored during immersion in SBF solution as shown in Fig.6.As soon as Mg is immersed in the SBF solution,corrosion takes place which results in the release of hydrogen gas and consequently formation of magnesium hydroxide,Mg(OH)2,on the surface of Mg according to Eq.(3).Since SBF solution contains aggressive ions such as chloride(Cl À),the Mg(OH)2layer can convert into soluble MgCl 2according to Eq.(4)[34].The soluble MgCl 2cannot protect the surface of Mg and consequently,the mass loss of the sample increases as the immersion time becomes longer resulting from magnesium dissolution.On the other hand,coated samples would be expected to lose mass due to both dissolution of the coating layer as well as corrosion of the magnesium once it is exposed to the electrolyte via either existing coating defects or coating dissolution.In order to estimate the maximum contribution to mass loss due to coating dissolution,the coating weight can be ing the theo-retical density of monetite and tricalcium phosphate based on their crystal structure parameters,the coating density can be estimated to be in the range of 2.9g/cm 3to 3.1g/cm 3.Taking the coating density as 3g/cm 3and assuming no gaps in the coatings based on the OCP results and previously reported coating morphology studies [22],this translates to approximately 3mg/cm 2coating weight for each 10m m coating thickness.Mg ðOH Þ2þ2Cl À/Mg ðCl Þ2þ2OH À(4)The total mass loss from the uncoated AZ31was 38.7mg/cm 2after 28days immersion in SBF solution,while that of the coated samples were signi ficantly lower.For example,samples coated at 160 C had mass loss of 14.3mg/cm 2compared to the estimated 18mg/cm 2original coating weight.In the case of samples coated at 100 C and 130 C,the mass loss after 28days immersion in SBF solution was 27.3mg/cm 2(estimated 6mg/cm 2original coating weight)and 21.4mg/cm 2(estimated 14mg/cm 2original coating weight),respectively,indicating that a signi ficantamountFig.6.Mass loss as a function of immersion time in SBF solution for uncoated and coatedsamples.Fig.7.Photographs of tested samples after 28days immersion test:a)AZ31substrate,b)HE100 C-3hr,c)HE130 C-3hr,d)HE160 C-3hr.S.Kaabi Falahieh Asl et al./Materials Chemistry and Physics 161(2015)185e 193190of mass loss must have been due to metal corrosion of these samples.The degradation rate of the magnesium samples in the 28days immersion test decreased three-fold as measured by the combined mass loss from corrosion and coating dissolution,when applying the coating at 160 C.The electrochemical tests indicated a much higher reduction of corrosion rate (approxi-mately 1000-fold for coating deposited at 160 C).However,it is necessary to consider the different time scales of the electro-chemical and immersion tests.While the electrochemical tests characterize the corrosion resistance in the presence of intact coating,the immersion tests take a much longer time apparently allowing some defects to develop on the coating and resulting in localized attack on the substrate as discussed in the next section.Once such localized coating defects occur,the corrosion can accelerate at the defect site allowing the eventual degradation of the metal substrate.In fact,such a behavior is desirable for implant application when the alloy is initially well protected from corrosion but it can degrade more readily once the bone healing process is under way.3.4.Surface morphology study after immersion testPhotographs of tested samples after 28days immersion in SBF are shown in Fig.7.As it can be seen,the uncoated Mg substrate undergoes severe and non-uniform corrosion.As immersion time increased to 28days,the pits on the surface of Mg became wider and started to penetrate through the substrate thickness.In some parts of the substrates,pits propagated through the whole 2mm thickness leading to premature loss of mechanical integrity.On the other hand,the samples coated at 130 C and 160 C displayed little change in the coatings and no obvious pits were observed on the coating surface.Since the Mg (OH)2layer formed from the metal is not protective and can transform into soluble MgCl 2,the breakdown of Mg(OH)2layer can provide more active sites which accelerates the substrate corrosion.Moreover,the difference in electrochemical activities of different phases in the alloy results in the formation of microcells and consequently galvanic corrosion can happen after dissolution of protective layers resulting in the formation of a non-uniform corrosion process [34].SEM images of the bare and coated samples after 28days im-mersion test are shown in Fig.8.It is clear that surface of uncoated substrate undergoes severe corrosion and shows a rough appear-ance.However,in the case of coated sample at 160 C,the coating layer still remained on the surface of substrate but Ca e P crystals started to dissolve as some pits appeared on their surfaces.Formation of pits on the surface of Ca e P crystals might be attrib-uted to the dissolution of monetite.Monetite is a stable phase below pH value around 5.Since the pH was around 7.2e 7.4in the SBF solution,this higher pH could break down the thermodynamic equilibrium at the interface of monetite and solution and results in dissolution of monetite phase according to Eq.(5)[35].CaHPO 4/Ca 2þþHPO 2À4(5)3.5.Ion release measurements and pH monitoring and evaluation of mechanical integrityThe ion release of uncoated and coated samples was measured by means of ICP techniques for magnesium,zinc and aluminum.The results are shown in Fig.9for magnesium and detailed in Table 3.Cumulative zinc and aluminum release did not exceed 0.2ppm/cm 2for the 28days immersion period.For the uncoated samples,the Mg ion release increased from 83ppm/cm 2after 2days immersion to 174ppm/cm 2after 28days immersion in SBF solution.The release of Mg ions from coated samples was low compared to the uncoated sample.In the case of samples coated at 160 C,Mg 2þrelease started from 11ppm/cm 2after 2days of im-mersion and reached 108ppm/cm 2after 28days immersion inSBFFig.8.SEM micrograph of:a)AZ31substrate,b)HE160 C-3hr after 28days immersion in SBFsolution.Fig.9.Mg ion release for uncoated and coated sample during immersion in SBF solution.S.Kaabi Falahieh Asl et al./Materials Chemistry and Physics 161(2015)185e 193191。

科罗索酸的抗肿瘤作用及其对CAM和YS M血管生成的影响李标;林伟民;田树红;李凡;李富【期刊名称】《中国生化药物杂志》【年(卷),期】2014(000)004【摘要】Objective To evaluate the antitumor effect of corosolic acid andits impact on CAM and YSM angiogenesis.Methods The effects of CRA onA549 proliferation was studied by MTT method in vitro.Tumor-bruden nude mice model were established by injecting A549 lung cancer cell marked with bioluminescent to nude mice,and the growth of tumor in mice were detected by IVIS small animal in vivo imagingsystem.Experiment of chicken embryos eggs are used to observe the roleof drugs on CAMand YSMblood vessels. Results The value of IC50 of CRA for A549 in vitro was 26.8μg/mL.A tumor-burdened animal experiment results showed that the CRA to A549 solid tumor has a certain therapeutic effect.CAM and YSM blood vessels of chicken embryos eggs treated with CRA were decreased significantly than negative control group.Conclusion CRA has certain therapeutic effect for A549,which may be related to the inhibition of angiogenesis in tumor tissues.%目的：探究科罗索酸（corosolic acid，CRA）的抗肿瘤作用及其对鸡胚绒毛尿囊膜（chick chorioallantoic membrane，CAM）和卵黄囊膜（yolk sac membrane，YSM）血管生成的影响。